How can I pre-set arguments in JavaScript function call? (Partial Function Application)

AlexH_40397 0
Viewed 26491

I am trying to write a JavaScript function that will return its first argument(function) with all the rest of its arguments as preset parameters to that function.

So:

function out(a, b) {
    document.write(a + " " + b);
}

function setter(...) {...}

setter(out, "hello")("world");
setter(out, "hello", "world")();

Would output "hello world" twice. for some implementation of setter

I ran into an issue with manipulating the arguments array on my first try, but it seems there would be a better way to do this.

functional-programming javascript
8 Answers

First of all, you need a partial - there is a difference between a partial and a curry - and here is all you need, without a framework:

function partial(func /*, 0..n args */) {
  var args = Array.prototype.slice.call(arguments, 1);
  return function() {
    var allArguments = args.concat(Array.prototype.slice.call(arguments));
    return func.apply(this, allArguments);
  };
}

Now, using your example, you can do exactly what you are after:

partial(out, "hello")("world");
partial(out, "hello", "world")();

// and here is my own extended example
var sayHelloTo = partial(out, "Hello");
sayHelloTo("World");
sayHelloTo("Alex");

The partial() function could be used to implement, but is not currying. Here is a quote from a blog post on the difference:

Where partial application takes a function and from it builds a function which takes fewer arguments, currying builds functions which take multiple arguments by composition of functions which each take a single argument.

Hope that helps.

JasonBunting_17900
user_88150

If you use Dojo you just call dojo.hitch() that does almost exactly what you want. Almost — because it can be used to pack the context as well. But your example is first:

dojo.hitch(out, "hello")("world");
dojo.hitch(out, "hello", "world")();

As well as:

var A = {
  sep: ", ",
  out: function(a, b){ console.log(a + this.sep + b); }
};

// using functions in context    
dojo.hitch(A, A.out, "hello")("world");
dojo.hitch(A, A.out, "hello", "world")();

// using names in context
dojo.hitch(A, "out", "hello")("world");
dojo.hitch(A, "out", "hello", "world")();

dojo.hitch() is the part of the Dojo Base, so as soon as you included dojo.js it is there for you.

Another general facility is available in dojox.lang.functional.curry module (documented in Functional fun in JavaScript with Dojo — just look on this page for "curry"). Specifically you may want to look at curry(), and partial().

curry() accumulates arguments (like in your example) but with one difference: as soon as the arity is satisfied it calls the function returning the value. Implementing your example:

df.curry(out)("hello")("world");
df.curry(out)("hello", "world");

Notice that the last line doesn't have "()" at the end — it is called automatically.

partial() allows to replace arguments at random:

df.partial(out, df.arg, "world")("hello");
Eugene_Lazutkin_263940

Easy way to achieve presetting of parameters without calling the function straight away (when e.g. awaiting user confirmation), is to "dress" the function in another anonymous function.

Instead of: exportFile(docType)

Do: function(){ return exportFile(docType) }

user_115667730

** EDIT: See Jason Bunting's response. This answer actually shows a sub-par way of chaining numerous out calls, not a single out-call with presets for some of the arguments. If this answer actually helps with a similar problem, you should be sure to make use of apply and call as Jason recommends, instead of the obscure way to use eval that I thought up. **

Well... your out will actually write "undefined" a lot in this... but this should be close to what you want:

function out(a, b) {
    document.write(a + " " + b);
}

function getArgString( args, start ) {
    var argStr = "";
    for( var i = start; i < args.length; i++ ) {
        if( argStr != "" ) {
            argStr = argStr + ", ";
        }
        argStr = argStr + "arguments[" + i + "]"
    }
    return argStr;
}

function setter(func) {
    var argStr = getArgString( arguments, 1 );
    eval( "func( " + argStr + ");" );
    var newSettter = function() {
        var argStr = getArgString( arguments, 0 );
        if( argStr == "" ) {
            argStr = "func";
        } else {
            argStr = "func, " + argStr;
        }
        return eval( "setter( " + argStr + ");" );
    }
    return newSettter;
}

setter(out, "hello")("world");
setter(out, "hello", "world")();

I'd probably move the code in getArgString into the setter function itself though... a little bit safer since I used 'eval's.

Illandril_178870

Using closures is another option. Use functions that return other functions!

user_91747280
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